Mass spectrometer



Oct. 1, 1957 K. P. LANNEAU MASS SPECTROMETER Filed June a, "1954 R G G .D. mT N R R TNm .HNE O AEF m mm r mm R EUU E UC NOW 7 A A w E R M MR .N .E U E ID N F C U AMN mw mm w E A D C E VIR YG A M m LE AR IE E m $6 2 m Rm SD l FIG. I

PEAK HEIGHT FREQUENCY (f) FIG. 2

KEITH R LANNEAUJNVENTOR BY/ 7 ATTORNEY United States Patent MASS SPECTROMETER Keith P. Lanneau, Baton Rouge, 1.21., assignor to Esso Research and Engineering Company, a corporation of Delaware This invention relates to mass spectrometry and more particularly relates to a method and means for focusing the ion beam of a radio frequency mass spectrometer for a selection of a predetermined mass peak. Still more particularly, this invention concerns a method and means for focusing the ion beam of a radio frequency mass spectrometer on individual mass peaks during a sequential selection of-different masses.

The application of mass spectrometry in the industrial field has become increasingly more important in recent years and the techniques of mass spectrometry have made possible accurate and rapid analyses of the components of a gaseous mixture. For example, mass spectrometers have been successfully utilized for continuous process monitoring, routine gas analysis, leak detection, and trace constituent analysis. In a mass spectrometer, the gaseous sample to be analyzed is introduced at'a low pressure to an ionization chamber wherein the gaseous molecules are ionized by means of a stream of electrons emitted from a hot wire filament. The various ions formed in the ionization chamber are then separated from each other on the basis of their mass/ charge (m/e) ratios in an analyzer section of the mass spectrometer to thereby measure the quantity of each mass present in the gaseous sample. This separation is accomplished by means of an electric field or a combination of an electric field and a magnetic field. By varying the strength of the electric and/or magnetic field, ions of a particular m/e may be made to strike a fixed target positioned in the path of the ions in the analyzer section. The impingement of the ion beam on this target causes a current to flow in a wire connected to the target and the magnitude of this current'is a measure .of the quantity of ions of the particular m/e selected. By determining the quantity of ions of .each m/e, an analysis of the gaseous sample is accomplished as is well known in the art.

In one particular type of mass spectrometer the ions formed in the ionization chamber are accelerated in the analyzer section by means of an electric field and are simultaneously bent from their normal linear path .by means of a magnetic field. Thus, for .a-given magnetic and electric field strength, the degree of curvature of the path of a given ion is a function of its m/e ratio. Normally in this type of mass spectrometer, a narrow slit is placed at a fixed distance along the path of the stream of ions so that for a given electric and/or magnetic field strength'it is possible to collect on the ion target in general only ions of a particular m/e ratio. Because of, this arrangement, it is possible to measure the quantity of ions of any particular m/e ratio by simply measuring the current flowing from the ion target fora preselected magnetic and electric field strength.

Another type of mass spectrometer is the radio frequency (R. F.) ion resonance mass spectrometer, which operates on a difierent principle thandQes the previously mentioned type of mass spectrometer. This latter type of mass spectrometer has been described inthe literature, such as in a paper presented at the American Chemical Patented Oct. 1, 1957 Society Symposium on Process Instrumentation, Chicago, Illinois, September 9, 1953, entitled An Ion Resonance Mass Spectrometer for Industrial Application by W. A; Morgan, G. Jernakofi and K. P. Lanneau. In the ion resonance mass spectrometer, the ionization chamber serves not only as a zone for ionizing the gaseous sample but also serves as the analyzer section of the mass spectrometer. This is accomplished by impressing a radio frequency electric field by means of electric voltage plates across the ionization chamber perpendicular to the electron beam and a magnetic field across the ionization chamber perpendicular to the electric field. The combi- 'nation of the electric and magnetic fields tends to move the ions formed in the ionization chamber in a spiral path in the plane of the electric field. For a particular set of conditions, namely, for a given electric and magnetic field, only ions of a particular m/e ratio will continue to be accelerated in a spiral path of increasing diameter. These ions have natural frequencies which correspond to the frequency of the' applied radiofrequency electric field, and are called resonant ions. Located at a fixed distance from the center of the ionization chamber in the plane of the ion path is an ion target upon which the resonant ions impinge. The other ions, which are termed non-resonant ions, will oscillate in the analyzer section due to the elfect of the electric and-magnetic fields but will never be accelerated sufiiciently to impinge upon the ion target. These non-resonant ions periodically spiral to a maximum radius or distance short of the ion target and then spiral back to the center of the analyzer section or tube as this section is sometimes called. The ion resonance principle is described in detail in the literature such as in the article in Physical Review of June 1, 1951, entitled The Measurement of e/m by .Cyclotron'Resonance, by H. A. Sommer, H. A. Thomas and J. A. Hipple.

The quantity and the m/ e ratio of ions striking the ion target is an indication of the composition of the gaseous sample. By varying the frequency of the electric field while maintaining constant magnetic field, and/or by varying the strength of the magnetic field while maintaiuing the frequency of the applied alternating electric field constant, ions having different natural frequencies, and hence different m/e ratios can be collected. The quantities of ions of different m/e ratios thus collected can be used as a measurement or indication .of the composition of the gaseous sample. This measurement-may be accomplished for example by determining the current produced by the ions striking the ion target. Because the ion current developed in the mass spectrometer is very small, it is normally amplifier, converted to a voltage,

and :the amplified voltage is then fed to a recorder.

One method by which a gaseous sample may be analyzed with this type of instrument is to uniformly vary the frequency of the electric field so as to provide a continuous scan of the spectrum of the sample. The'change of frequency may be accomplished by a number of different means in this type of analysis. The tuning capaci tor of'the radio frequency oscillator of the instrument may be rotated by mechanical movement by a motor drive on the capacitor for example. Another way of varying the frequency of the electric field in the case of an oscillator which is .of the inductance-capacitance tuned type is 'to change the inductance of the coil in the tank circuit. This may be accomplished by providing the coil with an extra winding through which a D. .0. .control current-may be passed. The control current may be supplied from a battery with the current being varied by a rheostat or the control current may be supplied from a D. C. sweep circuit for the continuous scan of the mass spectrum. In either case, such a spectrum will appear as a number of peaks on a plot of voltage against fre quency. The peak height representing the voltage measured by the recorder, of course, is a measure of the current flowing from the ion target which in turn is a measure of the quantity of ions of a given 121/ e ratio and the frequency is an indication of the particular m/ e ratio selected.

In another method of analyzing a gaseous sample, only a limited number of ion m/e ratios are selected to thereby reduce the time required for an analysis of the sample. In this method of analysis instead of uniformly varying the frequency of the radio frequency voltage a limited number of different frequencies are selected. The different frequencies may be selected in a number of different ways. For example, the oscillator for changing the frequency may be mechanically tuned by movement of the tuning capacitor. However, it is preferable to utilize an auxiliary electrical circuit which will change the inductance of an inductance-capacitance type of tuned oscillator similar to the circuit previously described for a continuous scan analysis. Thus, again the inductance of the coil in the tank circuit of the oscillator may be varied by providing the coil with an extra winding through which a direct current control current is passed. The current in the winding may be supplied from a battery and varied by a rheostat, although it is preferable to adjust the control current by means of a D. C. divider network through a stepper switch. Any of these means may thus be employed to accomplish a'sequential selection of individual masses. But when this is done, it is essential for accurate analysis to be sure that the frequency selected will properly focus on the top of the desired mass peak. Due to small variations in different parts of the mass spectrometer which normally occur during operation, there is usually no assurance that the top of the mass peak has been detected when employing a preselected frequency. As a result it is necessary to vary the frequency up and downslightly so as to scan the peak to make sure the peak has been measured.

The present invention concerns the problems associated vwith focusing the ion beam on individual mass peaks during a sequential selection of different m/e ratios. The present invention provides a method and means for focusing the ion beam so as to alleviate the difficulties normally associated with this type of analysis. Generally speaking, the present invention accomplishes this by modulating the frequency of the radio frequency voltage. More specifically the degree of modulation of the radio frequency voltage which is employed is a function of the width of the top portion of the mass peak (measured in terms of frequency) and is preferably selected to be somewhat less than the width (in frequency) of the top portion of the mass peak. This modulation causes the ion beam to fluctuate slightly with respect to the ion target so that a ripple having a frequency the same as the modulating frequency is produced in the ion current from the ion target. Generally it is preferable to have'the modulation band width approximately equal to the width of the top of the mass peak. The ripple in the ion current is converted to an alternating current by well-known electrical means and the alternating current is then amplified. In the preferred embodiment of the invention the phase of the amplified alternating current is then referenced by any well-known electrical means against the phase of the modulation of the radio frequency voltage and based on the particular phase and also on the amplitude of the amplified alternating current, a feedback signal is sent to the radio frequency oscillator to adjust the frequency of the radio frequency, voltage so that the A. C. component from the ion target approaches zero. When this occurs, the ion beam will be properly focused because then the frequency of the radio frequency voltage will be correct to hold in exact focus the top of the particular mass peak.

Thus an object of this invention is to provide a method and means for focusing directly on a mass peak during a sequential selection of different masses. Qther objects of this invention will be apparent from a reading of the specification which may be more readily understood by reference to the drawing in which:

Fig. 1 is a block diagram of an ion resonance mass spectrometer including a radio frequency oscillator, an ion current amplifier and a recorder, in combination with the apparatus of the present invention for focusing directly on a given mass peak; and

Fig. 2 is a diagrammatic plot of peak height against frequency illustrating the operation of the present invention.

Referring now to Fig. 1 reference character 10 designates a radio frequency (R. F.) ion resonance mass spectrometer. Reference character 11 designates an R. F. oscillator which is employed to produce and vary the frequency of the voltage of the R. F. electrical field in mass spectrometer 10 and reference character 12 designates an amplifier for converting the ion current to a voltage and amplifying the resultant voltage to provide mass peaks which are an indication of the quantity of ions of particular m/ e ratios. This amplified voltage is measured by recorder 13. The foregoing is conventional for R. F. ion resonance mass spectrometers.

In accordance with the present invention, the frequency of the R. F. voltage from oscillator 11 is modulated by frequency modulating signal device 14 The degree of modulation employed is preferably somewhat less than the width (in frequency) of the top portion of the mass peak. For example, a modulation corresponding to about 1050% of the width in frequency of the top portion of the mass peak measured at a point about 98% up the peak from the baseline is preferred. A frequency modulation in the range of about 0.05-1.0% generally will produce such results. It will be understood though that the peak width will vary somewhat depending upon the R. F. voltage iamplitude applied, i. e. depending upon the desired resolution. Because of the modulation of the R. F. voltage, the ion beam will fluctuate slightly on the ion target of mass spectrometer 10 and as a result instead of obtaining a steady direct current from the ion target, a direct current with a superimposed A. C. current will result. The modulation of the R. F. voltage, in terms of cycles per second, should preferably be held constant in a sequential selection of mass peaks. The frequency of the pulsating direct current from the ion target will thus be directly related to the frequency of modulating signal device 14 and an alternating current detector 15 is critically tuned to detect signals of this frequency.

Referring now to Fig. 2 a diagrammatic showing of one particular mass peak is illustrated. The top of the mass peak is designated by reference character A and a radio frequency of f, is required to produce point A. Suppose, however, that when mass spectrometer 10 was last standardized, a frequency was necessary to produce point A but since then due to minor changes in the elements of mass spectrometer 10 frequency 1, will now actually focus at point B on the curve of the mass peak. Therefore if frequency f, is selected as Wouldnormally be done, based on the last standardization, this would mean that a peak height as indicated by point B would then be detected by recorder 13 which would therefore actually be an erroneous indication of the amount of this particular ion actually produced from the gaseous sample. In accordance with the present invention, the modulation of the radio frequency produced by modulating signal device 12 causes a small fluctuation from point B as indicated by the arrows on the mass curve at point B. This small fluctuation produces the pulsating current from the ion target of mass spectrometer 10. The variation in frequency or modulation is designated by A) and the resultant variation in peak height is designated by APHi. From Fig. 2 it is clear that as f, is increased such that f, approaches t APHl will approach zero. The present invention is based upon this concept.

Referring again to Fig. 1, the pulsating voltage from amplifier 12 is converted to an A. C. ripple current in alternating current detector 15 and the resultant alternating current is then amplified in alternating current amplifier 16. The amplified alternating current is fed to phase discriminator and amplitude detector 17 wherein the phase of the amplified alternating current is referenced against the phase of the modulation from frequency modulating signal device 14 and then a feed back signal is sent to radio frequency oscillator 11 to correct its frequency so as to focus the ion beam from ion resonance mass spectrometer on the top of the mass peak. In accordance with the present invention the phase of the amplified alternating current from alternating current amplifier 16 determines whether the frequency of oscillator 11 will be increased or decreased, and the amplitude of the amplified alternating current determines the rate at which the frequency of oscillator 11 should be changed to minimize the error signal. Depending upon the particular type of ocillator employed the actual feedback signal to the oscillator could be used to control a servo motor attached to the shaft of the tuning capacitor of the oscillator or the feed back signal if in the form of a direct current could be fed back to a current controlled variable frequency oscillator if such type of oscillator should be employed.

It will be noted here that the present invention is able to detect the particular side of the peak upon which the mass spectrometer was focused prior to application of the corrective feedback. Thus referring again to Fig. 2 the present invention is able to detect whether the originally selected frequency is such that the resultant ion current is located say at point B or point C. For example, if frequency 1, had been orginally selected so that the peak height is represented by point B then the modulation to n a higher frequency from 1, would increase the peak height whereas on the other hand if frequency were originally selected such that the peak height would be at point C, a modulation increase from would result in a decrease in the peak height. The result therefore makes it possible for the present invention to detect the particular side of the peak upon which the mass reading has been orginally taken. For example, if frequency 7, were originally selected, the present invention would adjust the frequency of oscillator 11 upwards until APH1, or in other words the pulsation in the ion current approaches zero at which point the proper frequency of 1, would exist. Similarly, if frequency f, were orginally selected the present invention would decrease the frequency of oscillator 11 until APT-I2 approached zero at which time ion resonance mass spectrometer 10 would be properly focused at point A and oscillator 11 would be properly adjusted to f,,.

The present invention also contemplates producing the pulsation in the current from the ion target by a means other than modulating the frequency of radio frequency 6 oscillator 11. This other method consists of modulating the strength of the magentic field of mass spectrometer 10. This may be accomplished for example by means of Helmholtz coils wrapped around the pole faces of the permanent magnet which is employed in ion resonance mass spectrometer 10. In this invention then the modulating frequency can be applied either to the magnetic coils or to the R. F. oscillator.

Although it will be seen that some of the advantages of this invention can be obtained by frequency modulation without automatic feedback it is evident that the method is not as efiective as the combination of both modulation and feedback. The advantage, of course, to be gained by employing modulation alone is due to the resultant variation in frequency which is in the direction of the top of the mass peak which variation results in the detection of a point on the mass peak curve which is closer to the top of the peak than the point detected without modulation, the recorder in this case detecting the maximum ion current produced. However, in this case, use of modulation alone will result in reduced sensitivity and resolution as well as peak shape distortion.

What is claimed is:

1. In combination with a radio frequency ion resonance mass spectrometer wherein an ion beam is directed on an ion target to produce an ion current, means for fluctuating said ion beam on said ion target to thereby produce a pulsating ion current, means for measuring the amplitude of the pulsations of said ion current and for referencing the phase of the pulsations of said ion current against the phase of said fluctuating means, and means for centering said ion beam on said ion target, said centering means being controlled by said measuring and referencing means.

2. The combination of claim 1 in which said fluctuating means comprises a frequency modulation device.

3. A radio frequency ion resonance mass spectrometer adapted to measure maximum ion current at at least one selected mass peak comprising in combinaton: radio frequency acceleration means adapted to direct at least a portion of ions of particular mass in a spiral path to an ion target, alternating current modulation means adapted to vary the said portion of ions reaching the ion target, means for detecting alternating current variations in the resulting current at the ion target, and amplitude and phase discriminating feed-back means coupled to the radio frequency acceleration means adapted to adjust the frequency of said acceleration means to maximize the said portion of ions directed to the ion target.

Perkins Oct. 7, 1952 Washburn et al Ian. 27, 1953 

